diff --git a/src/sockaddr_str.c b/src/sockaddr_str.c
new file mode 100644
index 0000000..c9d9a94
--- /dev/null
+++ b/src/sockaddr_str.c
@@ -0,0 +1,378 @@
+/*! \file sockaddr_str.c
+ * Common implementation to store an IP address and port.
+ */
+/*
+ * (C) 2019 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ *
+ * Author: neels@hofmeyr.de
+ *
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ */
+
+#include "config.h"
+
+#ifdef HAVE_NETINET_IN_H
+#include <string.h>
+#include <errno.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+#include <osmocom/core/sockaddr_str.h>
+#include <osmocom/core/utils.h>
+#include <osmocom/core/byteswap.h>
+
+/*! \addtogroup sockaddr_str
+ *
+ * Common operations to store IP address as a char string along with a uint16_t port number.
+ *
+ * Convert IP address string to/from in_addr and in6_addr, with bounds checking and basic housekeeping.
+ *
+ * The initial purpose is to store and translate IP address info between GSM CC and MGCP protocols -- GSM mostly using
+ * 32-bit IPv4 addresses, and MGCP forwarding addresses as ASCII character strings.
+ *
+ * (At the time of writing, there are no immediate IPv6 users that come to mind, but it seemed appropriate to
+ * accommodate both address families from the start.)
+ *
+ * @{
+ * \file sockaddr_str.c
+ */
+
+/*! Return true if all elements of the osmo_sockaddr_str instance are set.
+ * \param[in] sockaddr_str  The instance to examine.
+ * \return True iff ip is nonempty, port is not 0 and af is set to either AF_INET or AF_INET6.
+ */
+bool osmo_sockaddr_str_is_set(const struct osmo_sockaddr_str *sockaddr_str)
+{
+	return *sockaddr_str->ip
+		&& sockaddr_str->port
+		&& (sockaddr_str->af == AF_INET || sockaddr_str->af == AF_INET6);
+}
+
+/*! Distinguish between valid IPv4 and IPv6 strings.
+ * This does not verify whether the string is a valid IP address; it assumes that the input is a valid IP address, and
+ * on that premise returns whether it is an IPv4 or IPv6 string, by looking for '.' and ':' characters.  It is safe to
+ * feed invalid address strings, but the return value is only guaranteed to be meaningful if the input was valid.
+ * \param[in] ip  Valid IP address string.
+ * \return AF_INET or AF_INET6, or AF_UNSPEC if neither '.' nor ':' are found in the string.
+ */
+int osmo_ip_str_type(const char *ip)
+{
+	if (!ip)
+		return AF_UNSPEC;
+	/* Could also be IPv4-mapped IPv6 format with both colons and dots: x:x:x:x:x:x:d.d.d.d */
+	if (strchr(ip, ':'))
+		return AF_INET6;
+	if (strchr(ip, '.'))
+		return AF_INET;
+	return AF_UNSPEC;
+}
+
+/*! Safely copy the given ip string to sockaddr_str, classify to AF_INET or AF_INET6, and set the port.
+ * Data will be written to sockaddr_str even if an error is returned.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] ip  Valid IP address string.
+ * \param[in] port  Port number.
+ * \return 0 on success, negative if copying the address string failed (e.g. too long), if the address family could
+ *         not be detected (i.e. if osmo_ip_str_type() returned AF_UNSPEC), or if sockaddr_str is NULL.
+ */
+int osmo_sockaddr_str_from_str(struct osmo_sockaddr_str *sockaddr_str, const char *ip, uint16_t port)
+{
+	int rc;
+	if (!sockaddr_str)
+		return -ENOSPC;
+	if (!ip)
+		ip = "";
+	*sockaddr_str = (struct osmo_sockaddr_str){
+		.af = osmo_ip_str_type(ip),
+		.port = port,
+	};
+	rc = osmo_strlcpy(sockaddr_str->ip, ip, sizeof(sockaddr_str->ip));
+	if (rc <= 0)
+		return -EIO;
+	if (rc >= sizeof(sockaddr_str->ip))
+		return -ENOSPC;
+	if (sockaddr_str->af == AF_UNSPEC)
+		return -EINVAL;
+	return 0;
+}
+
+/*! Convert IPv4 address to osmo_sockaddr_str, and set port.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] addr  IPv4 address data.
+ * \param[in] port  Port number.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_in_addr(struct osmo_sockaddr_str *sockaddr_str, const struct in_addr *addr, uint16_t port)
+{
+	if (!sockaddr_str)
+		return -ENOSPC;
+	*sockaddr_str = (struct osmo_sockaddr_str){
+		.af = AF_INET,
+		.port = port,
+	};
+	if (!inet_ntop(AF_INET, addr, sockaddr_str->ip, sizeof(sockaddr_str->ip)))
+		return -ENOSPC;
+	return 0;
+}
+
+/*! Convert IPv6 address to osmo_sockaddr_str, and set port.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] addr  IPv6 address data.
+ * \param[in] port  Port number.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_in6_addr(struct osmo_sockaddr_str *sockaddr_str, const struct in6_addr *addr, uint16_t port)
+{
+	if (!sockaddr_str)
+		return -ENOSPC;
+	*sockaddr_str = (struct osmo_sockaddr_str){
+		.af = AF_INET6,
+		.port = port,
+	};
+	if (!inet_ntop(AF_INET6, addr, sockaddr_str->ip, sizeof(sockaddr_str->ip)))
+		return -ENOSPC;
+	return 0;
+}
+
+/*! Convert IPv4 address from 32bit host-byte-order to osmo_sockaddr_str, and set port.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] addr  32bit IPv4 address data.
+ * \param[in] port  Port number.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_32(struct osmo_sockaddr_str *sockaddr_str, uint32_t ip, uint16_t port)
+{
+	struct in_addr addr;
+	if (!sockaddr_str)
+		return -ENOSPC;
+	addr.s_addr = ip;
+	return osmo_sockaddr_str_from_in_addr(sockaddr_str, &addr, port);
+}
+
+/*! Convert IPv4 address from 32bit network-byte-order to osmo_sockaddr_str, and set port.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] addr  32bit IPv4 address data.
+ * \param[in] port  Port number.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_32n(struct osmo_sockaddr_str *sockaddr_str, uint32_t ip, uint16_t port)
+{
+	if (!sockaddr_str)
+		return -ENOSPC;
+	return osmo_sockaddr_str_from_32(sockaddr_str, osmo_ntohl(ip), port);
+}
+
+/*! Convert IPv4 address and port to osmo_sockaddr_str.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] src  IPv4 address and port data.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_sockaddr_in(struct osmo_sockaddr_str *sockaddr_str, const struct sockaddr_in *src)
+{
+	if (!sockaddr_str)
+		return -ENOSPC;
+	if (!src)
+		return -EINVAL;
+	if (src->sin_family != AF_INET)
+		return -EINVAL;
+	return osmo_sockaddr_str_from_in_addr(sockaddr_str, &src->sin_addr, osmo_ntohs(src->sin_port));
+}
+
+/*! Convert IPv6 address and port to osmo_sockaddr_str.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] src  IPv6 address and port data.
+ * \return 0 on success, negative on error.
+ */
+int osmo_sockaddr_str_from_sockaddr_in6(struct osmo_sockaddr_str *sockaddr_str, const struct sockaddr_in6 *src)
+{
+	if (!sockaddr_str)
+		return -ENOSPC;
+	if (!src)
+		return -EINVAL;
+	if (src->sin6_family != AF_INET6)
+		return -EINVAL;
+	return osmo_sockaddr_str_from_in6_addr(sockaddr_str, &src->sin6_addr, osmo_ntohs(src->sin6_port));
+}
+
+/*! Convert IPv4 or IPv6 address and port to osmo_sockaddr_str.
+ * \param[out] sockaddr_str  The instance to copy to.
+ * \param[in] src  IPv4 or IPv6 address and port data.
+ * \return 0 on success, negative if src does not indicate AF_INET nor AF_INET6 (or if the conversion fails, which
+ *         should not be possible in practice).
+ */
+int osmo_sockaddr_str_from_sockaddr(struct osmo_sockaddr_str *sockaddr_str, const struct sockaddr_storage *src)
+{
+	const struct sockaddr_in *sin = (void*)src;
+	const struct sockaddr_in6 *sin6 = (void*)src;
+	if (!sockaddr_str)
+		return -ENOSPC;
+	if (!src)
+		return -EINVAL;
+	if (sin->sin_family == AF_INET)
+		return osmo_sockaddr_str_from_sockaddr_in(sockaddr_str, sin);
+	if (sin6->sin6_family == AF_INET6)
+		return osmo_sockaddr_str_from_sockaddr_in6(sockaddr_str, sin6);
+	return -EINVAL;
+}
+
+/*! Convert osmo_sockaddr_str address string to IPv4 address data.
+ * \param[in] sockaddr_str  The instance to convert the IP of.
+ * \param[out] dst  IPv4 address data to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv4 address string).
+ */
+int osmo_sockaddr_str_to_in_addr(const struct osmo_sockaddr_str *sockaddr_str, struct in_addr *dst)
+{
+	int rc;
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!dst)
+		return -ENOSPC;
+	if (sockaddr_str->af != AF_INET)
+		return -EAFNOSUPPORT;
+	rc = inet_pton(AF_INET, sockaddr_str->ip, dst);
+	if (rc != 1)
+		return -EINVAL;
+	return 0;
+}
+
+/*! Convert osmo_sockaddr_str address string to IPv6 address data.
+ * \param[in] sockaddr_str  The instance to convert the IP of.
+ * \param[out] dst  IPv6 address data to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv6 address string).
+ */
+int osmo_sockaddr_str_to_in6_addr(const struct osmo_sockaddr_str *sockaddr_str, struct in6_addr *dst)
+{
+	int rc;
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!dst)
+		return -ENOSPC;
+	if (sockaddr_str->af != AF_INET6)
+		return -EINVAL;
+	rc = inet_pton(AF_INET6, sockaddr_str->ip, dst);
+	if (rc != 1)
+		return -EINVAL;
+	return 0;
+}
+
+/*! Convert osmo_sockaddr_str address string to IPv4 address data in host-byte-order.
+ * \param[in] sockaddr_str  The instance to convert the IP of.
+ * \param[out] dst  IPv4 address data in 32bit host-byte-order format to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv4 address string).
+ */
+int osmo_sockaddr_str_to_32(const struct osmo_sockaddr_str *sockaddr_str, uint32_t *ip)
+{
+	int rc;
+	struct in_addr addr;
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!ip)
+		return -ENOSPC;
+	rc = osmo_sockaddr_str_to_in_addr(sockaddr_str, &addr);
+	if (rc)
+		return rc;
+	*ip = addr.s_addr;
+	return 0;
+}
+
+/*! Convert osmo_sockaddr_str address string to IPv4 address data in network-byte-order.
+ * \param[in] sockaddr_str  The instance to convert the IP of.
+ * \param[out] dst  IPv4 address data in 32bit network-byte-order format to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv4 address string).
+ */
+int osmo_sockaddr_str_to_32n(const struct osmo_sockaddr_str *sockaddr_str, uint32_t *ip)
+{
+	int rc;
+	uint32_t ip_h;
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!ip)
+		return -ENOSPC;
+	rc = osmo_sockaddr_str_to_32(sockaddr_str, &ip_h);
+	if (rc)
+		return rc;
+	*ip = osmo_htonl(ip_h);
+	return 0;
+}
+
+/*! Convert osmo_sockaddr_str address string and port to IPv4 address and port data.
+ * \param[in] sockaddr_str  The instance to convert the IP and port of.
+ * \param[out] dst  IPv4 address and port data to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv4 address string).
+ */
+int osmo_sockaddr_str_to_sockaddr_in(const struct osmo_sockaddr_str *sockaddr_str, struct sockaddr_in *dst)
+{
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!dst)
+		return -ENOSPC;
+	if (sockaddr_str->af != AF_INET)
+		return -EINVAL;
+	*dst = (struct sockaddr_in){
+		.sin_family = sockaddr_str->af,
+		.sin_port = osmo_htons(sockaddr_str->port),
+	};
+	return osmo_sockaddr_str_to_in_addr(sockaddr_str, &dst->sin_addr);
+}
+
+/*! Convert osmo_sockaddr_str address string and port to IPv6 address and port data.
+ * \param[in] sockaddr_str  The instance to convert the IP and port of.
+ * \param[out] dst  IPv6 address and port data to write to.
+ * \return 0 on success, negative on error (e.g. invalid IPv6 address string).
+ */
+int osmo_sockaddr_str_to_sockaddr_in6(const struct osmo_sockaddr_str *sockaddr_str, struct sockaddr_in6 *dst)
+{
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!dst)
+		return -ENOSPC;
+	if (sockaddr_str->af != AF_INET6)
+		return -EINVAL;
+	*dst = (struct sockaddr_in6){
+		.sin6_family = sockaddr_str->af,
+		.sin6_port = osmo_htons(sockaddr_str->port),
+	};
+	return osmo_sockaddr_str_to_in6_addr(sockaddr_str, &dst->sin6_addr);
+}
+
+/*! Convert osmo_sockaddr_str address string and port to IPv4 or IPv6 address and port data.
+ * Depending on sockaddr_str->af, dst will be handled as struct sockaddr_in or struct sockaddr_in6.
+ * \param[in] sockaddr_str  The instance to convert the IP and port of.
+ * \param[out] dst  IPv4/IPv6 address and port data to write to.
+ * \return 0 on success, negative on error (e.g. invalid IP address string for the family indicated by sockaddr_str->af).
+ */
+int osmo_sockaddr_str_to_sockaddr(const struct osmo_sockaddr_str *sockaddr_str, struct sockaddr_storage *dst)
+{
+	if (!sockaddr_str)
+		return -EINVAL;
+	if (!dst)
+		return -ENOSPC;
+	switch (sockaddr_str->af) {
+	case AF_INET:
+		return osmo_sockaddr_str_to_sockaddr_in(sockaddr_str, (void*)dst);
+	case AF_INET6:
+		return osmo_sockaddr_str_to_sockaddr_in6(sockaddr_str, (void*)dst);
+	default:
+		return -EINVAL;
+	}
+}
+
+/*! @} */
+#endif // HAVE_NETINET_IN_H